Can I swap my skeleton in my skeleton mesh?

I didn’t have a root bone for my skeleton. I bought a script that requires it so I then re-created my skeleton with a roo tbone. When I import it I cannot swap my skeleton mesh with the added root bone skeleton. Is there an alternative way to either add a root bone to my existing skeleton mesh or swap? When I right click my skeleton mesh and assign it the new skeleton I get a warning saying the my new skeleton has missing bones (when I just added a new one). The warning goes into a loop where nothing really gets adjusted.

Would appreciate any help. Thank you!